The present invention relates to a low noise filter, and in particular to a filter suitable for inclusion within an integrated circuit.
It is often desired to implement a filter function within an electronic circuit. Furthermore, with increasing trends towards miniaturisation it is generally desired that the implementation of the filter should be as physically small as possible. However, other considerations apply to filter design, a significant one of which is the noise generated by the filter.
An application in which filters must meet strict noise requirements is that of a phase locked loop (PLL) synthesiser within, for example, a mobile telephone. An example of an existing filter design for such an application is shown in FIG. 1 of the accompanying drawings. The filter, generally indicated 2, in FIG. 1 is,intended to be driven from a charge pump 4. The filter can be considered as generally comprising two cascaded stages. The first stage 6 comprises a capacitor 8 in series with a resistor 10 between an input node 12 and a ground connection 14. This series combination is shunted by a further capacitor 16. The second stage, generally indicated 20 follows the first stage and comprises a resistor 22 in series with a capacitor 24, and an output node taken from the connection between the resistor 22 and the capacitor 24.
The frequency response of the filter shown in FIG. 1 is schematically illustrated in FIG. 2. It can be seen that the amplitude of the output falls substantially uniformly over a first frequency range 26, which is primarily due to the action of the first stage 6. The rate of decrease then flattens off over a second region 28 corresponding to the magnitude of the impedance of the capacitor 8 now becoming equal to or smaller than the impedance of the resistor 10, and finally the attenuation increases again over a region 30. Within region 30 the attenuation increases for two regions. Firstly the impedance of capacitor 16 becomes low compared to the impedance of the resistor 10, and consequently the overall impedance of the first stage 6 of the filter begins to decrease again as a function of frequency. Secondly the impedance of the capacitor 24 in the second stage 20 of the filter becomes less than the impedance of the resistor 22.
Even though the components are passive, the resistors represent a source of thermally induced noise. This noise, known as Johnson noise can be expressed as:D(Vf2)=4k TRdfWhere D(Vf2) equals the mean square voltage fluctuations in a frequency range df
Where k is boltsmans constant,
T is absolute temperature,
R is resistance.
It is well known that for a single stage resistor-capacitor (RC) filter the break point occurs at       1          2      ⁢                           ⁢      π      ⁢                           ⁢      RC        .Referring to FIG. 1 again, the capacitor 8 has a value of 27 nanofarads which is so large that it would not be economical to fabricate it on an integrated circuit because of the amount of the die area which would be required. Furthermore, if the capacitor is external to the integrated circuit which contains the associated circuitry, pins will be required in order to connect the capacitor which may incur an increased packaging cost. An external capacitor of that value and of sufficient accuracy and stability for use in a PLL filter has a cost which may be a significant proportion of the total cost of the PLL. It is desirable to reduce the number of external components because physical size and assembly cost are significant factors in the design of many products. For all of these reasons it would be desirable for the capacitor values in the filter to be reduced sufficiently that all of the components could be included in an integrated circuit. If all of the capacitor values in an RC filter are decreased by a given factor X, the impedance of those capacitors at all frequencies will increase by the same factor X, and in order for the output of the filter to remain the same as the original circuit, all of the resistors must be increased by the factor X as well as any current sources being reduced by the factor X. If a resistor is increased by the factor X, the Johnson noise voltage of that resistor will increase by a factor equal to the square root of X. The noise of the original filter circuit of FIG. 1 may not be increased without degrading the performance of the circuit in which it operates, so a circuit having the form of FIG. 1 is not suitable for fabrication within an integrated circuit.